1. Field of the Invention
The present invention generally relates to semiconductor manufacturing processes and more particularly to a removing impurities from insulating layers during semiconductor chip manufacturing.
2. Background Description
Silicon device reliability degradation resulting from mobile charge Q.sub.m in the device structure is a well known problem. Alkali mobile ions, e.g., Na.sup.+, Li.sup.+ or K.sup.+, are typical sources of Q.sub.m. These alkali ions contaminate critical device oxide, particularly the gate oxide and, also, oxide covering the device perimeter. The ionic contamination results in mobile charges in the oxide.
The mobility of these ionic impurities is affected by bias and temperature, i.e., typical device operating conditions. Over time, device operation causes mobile ions to move through the oxide causing operational device functional characteristic variations, such as device threshold voltage shifts, subthreshold leakage and impaired device isolation. Circuits with affected devices can become unstable.
There are any number of steps in state of the art manufacturing processes that may be the source of these mobile alkali ions: oxidation steps, wet chemical process steps, reactive ion etching (RIE), photolithographic steps, chemical-mechanical-polish (CMP) steps, or chemical vapor deposition (CVD) process steps. Preventing ionic contamination from these alkali mobile ions is very difficult and expensive.
FIGS. 1A-C show mobile ions being formed in a cross-section of a semiconductor wafer during typical prior art semiconductor manufacturing steps, field effect transistor (FET) manufacturing steps in this example. In FIG. 1A, a semiconductor layer 50 has device regions 52 defined by thick isolation insulator regions 54. A thin gate oxide layer 56 is formed over the semiconductor layer 50.
FIG. 1B is an expanded view at a typical device region 52. In the expanded view of FIG. 1B, mobile ions 58 (Q.sub.m) are trapped in the thick isolation insulator 54 and thin gate oxide 56. In this example, the mobile ions 58 are primarily positively charged ions, and primarily at the semiconductor-isolator interface, e.g., the oxide-silicon interface. Generally, mobile ions introduced at a given process step may locate at any depth in the dielectric. Thus, the structure of FIG. 1B shows a typical semiconductor wafer after gate oxide formation. Mobile ions 58 remain in the structure as a result of prior processing steps.
In FIG. 1C, a gate 60, e.g., polysilicon, is formed on the structure of FIG. 1B. The gate oxide layer 56 is etched away from thick isolation regions 54, leaving gate oxide 56' under gate 60. Mobile ions 58 remain in gate oxide 56' and in thick insulator regions 54.
Under normal device operation, bias voltages on the gate 60 eventually force the mobile ions 58 through the gate oxide 56'. The movement of these mobile ions 58 alters the FET's threshold voltage (V.sub.T), making the FET unstable, in effect, giving the FET a time varying V.sub.T. If the polysilicon gate bias voltage is positive, the mobile ions move towards the channel, shifting the V.sub.T more negative or, less positive and, vice versa.
Typical prior art approaches to resolving this mobile charge problem and its associated reliability degradation has been through controlling device contamination with wafer monitors and product kerf testing. However, because process monitors are expensive, only selective process tools are monitored.
Further, the lag between introduction of unwanted ions and discovering their existence from Q.sub.m test information is, typically, several days to weeks later, after the chip manufacturing process is nearly complete. In typical 0.5 .mu.m technologies, the earliest kerf in-line test probe occurs at the end of device definition, after forming a first metal wiring layer. This can be well after ionic contamination. At this point, an entire lot, partially completed, may be scrapped due to this ionic contamination.
Thus, there is a need for methods for reducing mobile ion contamination in semiconductor manufacturing processes.